1. Field of the Invention
The present invention relates lo a brush-use DC (direct current) motor, and more particularly relates to a compact DC motor for use in a camera, etc.
2. Discussion of the Background
In a bush-use DC motor, a rotor in which rotor coils are wound is fixed on a rotation shaft. A stator applies magnetic field to the rotor via magnetic poles of the stator opposing magnetic poles of the rotor. Further, a DC drive current is switched corresponding to a rotation angle of the rotor and is applied to the rotor coils through a commutator, which rotates together with the rotor and through an electrode brush in sliding contact with the commutator.
The commutator includes contact electrode parts integrally provided with the rotation shaft. The contact electrode parts include plural segments and are connected to the rotor coils.
In addition, a DC drive current is fed from a DC power supply to the electrode brush in sliding contact with the contact electrode parts of the commutator, and is then applied to the rotor coils while the flow direction of the DC drive current is switched by the commutator. Specifically, the flow direction of the DC drive current applied to the electrode brush is switched when the electrode brush switches from one segment to another segment of the contact electrode parts due to the rotation of the commutator.
Generally, the commutator employed in the DC motor of this type includes a circular cylinder-shape, and the cylindrical surface of the commutator is divided into plural contact electrode parts which surround the circumference of the rotation shaft at equally angled intervals with a small gap separating each part. In addition, respective contact electrode parts connect to the rotor coils. A pair of electrode brushes connected to the DC drive power supply is brought into sliding contact with the contact electrode parts of the commutator on rotation angle position of, for example, 180° relative to the commutator.
This type of the background brush-use DC motor is generally constructed such that a pair of electrode brushes is in sliding contact with the above-described contact electrode parts formed on the cylindrical surface of the commutator while pressing toward the axial line of the rotation shaft (i.e., in substantially the radial direction).
For example, FIG. 17 illustrates a three-pole brush-use DC motor. In the three-pole motor, electricity is fed to a commutator CM0 which is in sliding contact with a pair of electrode brushes B01 and B02 from a DC drive power supply E0 through the paired electrode brushes B01 and B02. The paired electrode brushes B01 and B02 are brought into contact with the commutator CM0 an rotation angle positions different by 180°. The commutator CM0 includes three pieces which form a cylindrical surface and rotates together with a rotor of the DC motor. The three pieces of the commutator CM0 are separated at equally angled intervals of about 120°.
Further, three rotor coils are connected to each other between the adjacent pieces of the commutator CM0, and thereby three rotor magnetic poles are formed therebetween. The polarity of these rotor magnetic poles varies depending on the contact state of each piece of the commutator CM0 and the electrode brushes B01 and B02 which changes corresponding to the rotation angle of the rotor. Thereby, a rotation driving force is generated between, for example, a pair of stator magnetic poles of a permanent magnet at the side of a stator (not shown).
With the rotation of the rotor, respective rotor magnetic poles oppose respective stator magnetic poles in order, and the contact state of each piece of the commutator CM0 and the electrode brushes B01 and B02 changes. Thus, by varying the polarity of each rotor magnetic pole in order, the rotor continually rotates.
Specifically, when a voltage is applied to the paired electrode brushes B01 and B02 from the power supply E0, the current flows from one of the electrode brushes B01 and B02 to the other through the rotor coils. The magnetic field is generated by the rotor coils, and thereby the rotor magnetic poles are formed. By the action of the magnetic field generated by the rotor coils and the magnetic field generated by the stator magnetic poles, the rotor rotates.
As a method of detecting the rotation of the above-described motor, a rotary encoder method is known. Specifically, in the rotary encoder method, a rotation slit disk having slits on the circumferential surface thereof is provided on a rotation output shaft of the motor or in a power transmission mechanism rotated by the rotation output shaft. The rotation of the motor is detected by detecting the slits on the circumferential surface of the rotation slit disk with a photointerrupter. Although the rotary encoder method allows an accurate detection of the rotation of the motor, the space and cost for the rotary encoder constructed by the rotation slit disk and the photointerrupter are inevitably increased.
Further, in the above-described brush-use DC motor in which the electrode brushes are brought into sliding contact with the cylindrical surface of the commutator while pressing toward the axial line of the rotation shaft, assembling the commutator and the electrode brushes is typically difficult in the DC motor.
Specifically, in this type of DC motor, because the electrode brushes are configured to press the cylindrical surface of the commutator, the brush-to-brush spatial distance is narrower than the diameter of the commutator. For this reason, in assembling the motor, the commutator needs to be inserted in the narrow space between the electrode brushes. This may cause the deformation of the electrode brushes due to contact with the commutator during assembly.
Further, to avoid the positional deviation of the electrode brush in the axial direction of the rotation shaft and to avoid the electrode brush falling from the circumference surface of the commutator when the DC motor rotates, flanges of a relatively large diameter may be formed at both ends of the cylinder-shaped commutator. These flanges also make assembling the DC motor difficult.
Moreover, in the above-described DC motor, because the commutator has a cylinder shape, the commutator occupies a larger space expanding in the thrust direction along the axis of the rotation shaft of the DC motor.